1. FIELD OF THE INVENTION
The present invention relates to fan wheels of centrifugal and axial fans such as blowers, exhaust fans and the like, impellers of pumps, steam and gas turbine rotors and fan wheels attached to generators and motors for cooling them and more particularly wheels or rotors with a plurality of blades especially adapted for use in rotary machines which must be operated with less noise and less vibrations.
2. BRIEF DESCRIPTION OF THE PRIOR ART
When a fan wheel is rotated, the noise is produced from the fan wheel itself or from a machine incorporating the fan wheel. Most unpleasant is the fan noise associated with the number of blades. In order to control the fan noise, there has been proposed a fan wheel or the like wherein a plurality of blades are arranged at irregular or unequal pitch angles. However the irregular or unequal pitch angles have been determined mainly in dependence upon the intuition and no satisfactory theory has been established which may obtain an optimum set of irregular or unequal pitch angles of blades so that the fan noise may be suppressed to a minimum level. In addition, the irregular or unequal pitch angle arrangement of blades inevitably gives rise to the inherent problem how to attain the mechanical balance of the fan wheel at the same time. Again so far no satisfactory theory to solve this problem has been proposed yet.
For instance, consider a blade arrangement wherein 12 blades are divided into four sets each consisting of three blades spaced equiangularly and the adjacent sets are circumferentially spaced apart from each other by a suitable angle, whereby the blades may be arranged at irregular or unequal pitch angles. This arrangement serves to attain the mechanical balance of the fan wheel, but will not suppress the unpleasant fan noise sufficiently because each set of three blades would act as an equally pitched fan. Thus it has been extremely difficult to attain a compromise between the irregular or unequal pitch angle arrangement of blades and the attainment of satisfactory mechanical balance of the fan wheel.